No sound on some movies?

I have been using my external Hard Drive to play movies that I have downloaded and it worked fine but recently the movies I download have no sound when I play them on my PS3 but play fine with my computer. Is there something I need to do to get the sound or a codec or something? Thanks

Thanks, I will take a look at that but I am having trouble on avi, mp4 and divx files that I have had no problems in the past. I know that computers have to be updated with codecs and so on so I wondered if the PS3 had some updates or is there something else?